Optical devices, for example, OLEDs, may be categorized into a bottom emission structure in which light is emitted toward a glass substrate, and a top emission structure in which light is emitted in a direction opposite to a glass substrate, depending on the emission structure. In the bottom emission structure, a cathode acts as a reflector by using a metal thin film of aluminum or the like, and an anode acts as a path, through which light is emitted, by using a transparent oxide conductive film of indium tin oxide (ITO) or the like. In the top emission structure, a cathode is formed as a multi-layered thin film including a very thin silver thin film, and light is emitted through the cathode. In the field of lighting panels, except for transparent panels in which light is emitted through both surfaces, the bottom emission structure is generally used, with the top emission structure being rarely used.
In a laminate used for an optical device, such as an OLED, only about 20% of emitted light is emitted to the outside, and about 80% of the emitted light is lost. There are two grounds for the loss of light: (1) a wave-guiding effect due to a difference in refractive indexes among a glass substrate, a transparent electrode and an organic layer, and (2) a total reflection effect due to a difference in refractive indexes between the glass substrate and air.
This is because a planar waveguide is naturally formed in the OLED due to conditions wherein a refractive index of an internal organic layer is about 1.7 to 1.8, a refractive index of ITO generally used as a transparent electrode is about 1.9, a thickness of the two layers is about 200 nm to 400 nm (very thin), and a refractive index of the glass used as a substrate is about 1.5. Calculation shows that an amount of light lost by the wave-guiding effect is about 45% of the emitted light.
Light extraction technology is progressively drawing much attention as a core technology that increases efficiency, luminance, and service life of optical devices. Technology of extracting light isolated between an organic layer and electrode is called the internal light extraction technology.
According to reported research, an internal light scattering layer, deformation of a substrate surface, a refractive index adjustment layer, photonic crystals, a nanostructure forming method, etc., are known to be effective on extraction of internal light. A main objective of the internal light extraction technology is to scatter, diffract, or refract the light isolated due to the wave-guiding effect in order to form an incident angle less than or equal to the critical angle, thereby extracting light to the outside of an optical waveguide.
Patent Document 1 discloses an internal light extraction layer having a structure wherein light scattering nanoparticles are applied on a substrate of low refractive index with one-dimensionally or two-dimensionally periodical structures formed thereon, and a flattening layer of high refractive index is then applied thereto.
Patent Document 2 discloses an internal light extraction layer having a structure wherein a layer having periodical nanostructures is formed on a substrate of low refractive index by using a printing process, such as imprinting (an additional scattering element may be included), and a flattening layer of high refractive index is then applied thereto.
Patent Document 3 discloses an internal light extraction layer wherein concavo-convex structures are formed on a substrate and does not comprise a flattening layer.
The processes described in the references above are not suitable for producing optical devices such as OLED in a large area scale.
Patent Document 4 discloses an internal light extraction layer having a structure wherein a surface of a substrate is roughened or a film having a micro-structure is attached on a surface of a substrate having low refractive index, and a flattening layer having high refractive index is then applied thereto. Micro-structured film is formed by casting a photopolymer on PET film, which is then filled with polymer. Finally, double layers of 3M Laminating Adhesive 8141 are laminated thereon.
In Patent Document 4, the material used for the formation of nano-structures via a patterning process is primarily a polymer or an organic binder. However, using a patterning process is still problematic in that the polymer or organic binder may be decomposed to cause an outgassing phenomenon and stabilization of the shape of the nano-structures may not be maintained during the subsequent high temperature processes.
Patent Document 5 discloses formation of a structured layer which has feature sizes in the range of several micrometers to scores of μm regularly using the imprinting method, in which a stamp or roller is used. An intermediate layer may be deposited using a liquid solution onto the carrier body in the subsequent planarization step, to reduce the average roughness of the surface of the carrier body.
However, Patent Document 5 does not limit the thickness of the intermediate layer and does not describe the roughness of the intermediate layer for avoiding the problem of short circuit between electrodes. Furthermore, an additional patterning process, such as imprinting, is needed to make the structured layer.
Patent Document 6 discloses formation of a convex structure by using a glass flit paste, wherein the width of the convex structure is limited to 200 μm.
In Patent Document 6, the pitch between the convex structures that can be stably formed is at least about 200 μm, since a pattern having a high inclination angle cannot be formed considering the upper limit of thickness of the flattening layer. Further, a height of the convex structure is limited to the range of 5 μm to 200 μm in order to obtain a predetermined light extraction effect.
If the convex structure has a height of less than 8.75 μm, a low inclination angle (about 5 degrees) is shown to be ineffective in light extraction (i.e., 26.5%). Therefore, to form a convex structure having high inclination angle (about 15 degrees), the height of the convex structures should be more than 26.79 μm. There still remains the problem, however, that forming stable convex structures having a high inclination angle requires the thickness of a flattening layer to be at least double the height of the convex structures to fully cover the convex structures.
Further, when glass frit is used as raw material for the flattening layer, the problem that the concentration of captured air bubbles generated during sintering of the glass frit increases as the thickness increases was found. This means that the flattering layer must have a thickness which fully covers the convex structures. In this regard, there still remains the risk of air bubbles being captured in the internal light extraction layer during sintering, which results in the loss of optical properties of the light emitting device due to increase of light paths.
Therefore, there still remains a need for a simple and economic process for preparing an internal light extraction layer made of enamel (melted glass frit) comprising reliefs. Also needed is an internal light extraction layer made of enamel which can efficiently discharge light to the exterior. Further, there still remains a need for a preferred process for forming a concavo-convex structure without applying an additional patterning process, since applying a patterning process onto a glass substrate is not the most suitable means for producing optical devices, such as OLEDs, at large scale.